First Summer 2008

We started with a simple exercise.

These are three ways to swap the contents of two integer variables:

n = 3
m = 5

print n, m

temp = n
n = m
m = temp

print n, m

(n, m) = (m, n)

print n, m

n = n + m
m = n - m
n = n - m

print n, m

• ingredients,
• local utensils (variables) and
• produces (returns) results.

So we discussed this code:

def fun(x):
    x = x + 1
    print "x inside the function:", x
    
x = 3
fun(5)
print "x outside the function:", x

def fun(x):
    x = x + 1
    print "x inside the function:", x
    
x = 3
fun(x)
print "x outside the function:", x

n = 3
m = n
m = m + 1
print n

n = [3, 1, 7, 5, 2]
m = n
m.append(12)
print n

The address is like a number. In the first example the number in n was copied in m.

Same thing happens here except this time the number is an address (to a list).

Next we decided we wanted to define a procedure to sort a list of integers in ascending order.

We didn't know how, so we settled for a lower target.

We wrote a procedure that was determining if a list was sorted or not:

def sorted(a):
    for i in range(len(a)-1):
        if a[i] > a[i+1]:
            return False
    return True

numbers = [4, 7, 3, 1, 5]
print sorted(numbers)
numbers = [1, 2, 4, 5, 9]
print sorted(numbers)

So we ended up designing an algorithm for which the code was to be written tomorrow:

def sorted(a):
    for i in range(len(a)-1):
        if a[i] > a[i+1]:
            return False
    return True

def sort(a):
    while not sorted(a):
        for i in range(len(a)-1):
            if a[i] > a[i+1]:
                (a[i+1], a[i]) = (a[i], a[i+1])

>>> numbers = [3, 11, 7, 5, 12, 2]
>>> sort(numbers)
>>> numbers
[2, 3, 5, 7, 11, 12]
>>>